1. Field of the Invention
The present invention relates to surface acoustic wave apparatuses including a surface acoustic wave filter having a balanced-to-unbalanced conversion function.
2. Description of the Related Art
Remarkable technical progress has been made on portable telephones (communication apparatuses) in terms of their compactness and light weight. To achieve this, the number of components has been reduced, the size of components has been reduced, and components having a plurality of functions have been developed.
Surface acoustic wave apparatuses having a balanced-to-unbalanced conversion function, a so-called balun function, used in RF stages of portable telephones have been actively researched, and have been primarily used in global systems for mobile communications (GSMs).
Several patent applications relating to surface acoustic wave apparatuses having a balanced-to-unbalanced conversion function have been filed. FIG. 3 shows a surface acoustic wave apparatus having a balanced-to-unbalanced conversion function with the impedance of an unbalanced signal terminal side set to 50 Ω and the impedance of a balanced signal terminal side set to 200Ω, which is disclosed in Japanese Unexamined Patent Publication No. JP 11-97966.
In the structure of FIG. 3, in a longitudinally-coupled-resonator-type surface acoustic wave filter 301 in which three interdigital transducers (hereinafter called IDTs) are arranged in direction in which surface acoustic waves propagate, an IDT 303 disposed at the center is divided into two portions substantially symmetrically in the propagation direction of the surface acoustic waves, and the two portions are connected to balanced signal terminals 308 and 309, and left and right IDTs 302 and 304 having inverted polarities are connected to an unbalanced signal terminal 307. With this structure, a balanced-to-unbalanced conversion function is provided by the inverted polarities, and the impedance at the balanced signal terminal side is about four times as high as that of the unbalanced signal terminal side by the division of the IDT 303 into the two portions.
Filters having a balanced-to-unbalanced conversion function must have equal amplitude characteristics and phases that are inverted by 180 degrees in the transfer characteristics at a pass band between an unbalanced signal terminal and balanced signal terminals. They are called an amplitude-balance degree and a phase-balance degree.
The amplitude-balance degree and the phase-balance degree are defined in the following manner assuming that filter apparatuses having a balanced-to-unbalanced conversion function are three-port devices. The amplitude-balance degree=|A|, A=|20 log(S21)|−|20 log(S31)|, the phase-balance degree=|B−180|, B=|∠S21−∠S31|, where an unbalanced input terminal is called port 1, and balanced output terminals are called port 2 and port 3. Ideally, the amplitude-balance degree should be 0 dB and the phase balance degree should be 0 degrees in the pass band of surface acoustic wave filters.
However, the balance degrees of the conventional structure shown in FIG. 3 are insufficient. This is because the electrode fingers (310 and 317 in FIG. 3) of the IDT 302 and the IDT 304, adjacent to the IDT 303 have different polarities from each other, and therefore, parasitic capacitances and bridging capacitances differ at the balanced signal terminals 308 and 309.